Thermal Responsiveness of 1,2,4-Triazolium-Based Poly(ionic liquid)s and Their Applications in Dye Extraction and Smart Switch

Triazoliums are a family of five-membered heterocyclic cations that contain three nitrogen and two carbon atoms. In contrast to the widely studied imidazolium cations, triazoliums are less explored. In terms of the chemical structure, triazolium replaces a carbon atom in the imidazolium cation ring with an electron-withdrawing nitrogen atom, which makes the triazolium more polarized. Among the many physical properties, the thermal responsiveness of triazoliums is of particular interest to us but has been rarely investigated. In this contribution, we prepared a series of 1,2,4-triazolium-based poly(ionic liquid)s (PILs) with varying alkyl substituents and counteranions and studied their thermal-responsive behavior. We found that 1,2,4-triazolim-based PILs with a polymeric backbone structure similar to that of polyimidazoliums exhibited opposite thermal phase transition processes in solvents. For example, methyl-substituted 1,2,4-triazolium-based PILs exhibited an upper-critical-solution-temperature (UCST)-type phase transition in methanol when the counterion was I^- and a lower-critical-solution-temperature (LCST)-type phase transition in acetone when the counterion was PF₆ ^-. The thermal responsiveness was reversible and concentration-dependent. Interestingly, the thermal response of 1,2,4-triazolim-based PILs could be retained in the organogel form, which was applied in the pretreatment of anion-containing organic waste liquids and temperature-controlled "smart" switches.